Certain methacrylate-functional silanes have been known, including, for example, 3-methacryloxypropyltrimethoxysilane, which has uses in glass fiber-reinforced composites of various thermoplastics and in durable coatings for metallic substrates. There has been a continuing need for products with differentiated and improved performance in areas in which 3-methacryloxypropyltrimethoxysilane is used, whereby differences in molecular structure result in useful differences in silane performance in properties as diverse as rate of hydrolysis, aqueous solubility, low product color, processability of the substrate being treated from aspects of lubricity, static control, fiber strength, speed of fiber drawing, and the like. One class of structural variations which has become useful relative to 3-methacryloxypropyltrimethoxysilane is the class of methacrylamidoalkylalkoxysilanes. Prior to the present invention, the processes for the production of acrylamidoalkylalkoxysilanes have had at least one major deficiency.
One early disclosure for the preparation of acrylamidoalkylalkoxysilanes is U.S. Pat. No. 3,249,461, wherein aminopropyltrimethoxysilane is reacted with methacryloyl or acryloyl chloride in an inert solvent blend. The reaction generates an equimolar amount of hydrogen chloride by-product, which was removed by washing the product/by-product mixture with excess aqueous sodium carbonate. Deficiencies of this process include the use of solvents, which lowers yield based on unit volume of production equipment, the formation of hydrogen chloride by-product necessitating its removal, and the removal of hydrogen chloride by aqueous washing, which will hydrolyze a significant portion of the methoxysilane groups forming higher oligomeric or polymeric siloxanes.
The above process has been modified in U.S. Pat. No. 4,711,943 wherein the hydrogen chloride by-product has been handled by formation of tertiary amine hydrochloride salt, which can be removed by filtration or centrifugation. Solvent is still used, and the formation and removal of solid tertiary amine hydrochloride salts further reduces yield per unit volume, adds a difficult and time-consuming step, and generates an undesirable waste which must be disposed of or treated for recycle. U.S. Pat. No. 3,249,461 also discloses that the corresponding acid anhydride may be used in place of the acid chloride, and U.S. Pat. No. 3,900,679 discloses formation of methacrylamidoalkylalkoxysilanes by reaction of aminoalkylsilanes with methacrylic acid. Reaction of amines with acids, which would also be by-products of the acid anhydride route above, to form amide groups is accompanied by the formation of an equimolar amount of water. As is known in the art, water will hydrolyze two equivalent amounts of the alkoxysilane groups, forming siloxanes, which reduce product purity, and build molecular weight and viscosity, potentially to the point of gelation.
U.S. Pat. No. 4,990,641 teaches a hydrosilation route to structurally different methacrylamidoalkyl-bis-alkoxysilanes as well as an acid chloride route to similar structures. The hydrosilation route to methacrylamidoalkylalkoxysilanes may be complicated by cyclizations of the N-allyl(meth)acrylamide starting materials under reaction conditions (Chem. Abstr., 95, 94175r(1976)). Lastly, the reaction of silazacyclobutanes with acid chlorides is disclosed in U.S. Pat. No. 5,446,180 and the reaction of acid chlorides with silazacyclopentanes in U.S. Pat. No. 5,082,958. This approach is practical only for monofunctional alkoxysilanes, which are of less utility than the di- and trialkoxysilane derivatives. It also requires the intermediate preparation of the commercially unavailable silazacycloalkanes. Thus, while utility for methacrylamidoalkylalkoxysilanes has been increasing (see U.S. Pat. Nos. 4,243,426, 4,762,759, 5,008,349, and 5,372,841 among others), there is still a need for improved processes to make said methacrylamidoalkylalkoxysilanes.
While the reaction of aminoalkylsilanes with acrylate esters is known as disclosed in U.S. Pat. No. 4,209,455, which is incorporated herein by reference, the reaction conditions are such that Michael addition of the amine group to the acrylate double bond is the first reaction which occurs, and the products are devoid of (meth)acrylate functionality.